The use of amplified wavelength-division-multiplexed (WDM) communication systems can substantially increase the transmission capacity of existing single-mode fiber systems. However, WDM systems require strict spectral control of the transmitter lasers in order to maintain constant channel spacing and avoid crosstalk. Thus, at a minimum, each laser should operate at one specific wavelength for the entire lifetime of the system. If this were the only criterion, every WDM system (which may be part of a large scale WDM network) could operate at a different set of wavelengths. Since in this case each laser would operate at one particular preselected wavelength that may be different from all the other lasers, it would be necessary to accurately track and record the operating wavelength of every transmitter laser in the network. Moreover, due to the lack of standardization, a large stock of replacement lasers would be required since no single replacement could substitute for all the lasers employed in the system. These deficiencies could be eliminated if all the WDM transmitter lasers operated at the same predetermined set of wavelengths. In addition, such standardization could increase the multivendor compatibility of WDM transmission equipment.
Known techniques for maintaining a constant channel spacing in a WDM system include those disclosed in B. S. Glance et al., "Densely Spaced FDM Coherent Optical Star Network With Optical Signals Confined To Equally Spaced Frequencies," J. Lightwave Technol., vol. 6, pp. 1770-1781, Nov. 1988, and K. Nosu et al., "Optical FDM Transmission Technique," J. Lightwave Technol., vol. LT-5, pp. 1301-1308, Sept. 1987. In these systems the transmitter lasers are locked to an optical resonator. Absolute frequency references such as lasers frequency-locked to an atomic or molecular absorption line have been added to these resonators to ensure long-term stability (see Y. C. Chung et al., "WDM Coherent Star Network With Absolute Reference," Electron. Lett., vol. 24, no. 21, pp. 1313-1314, 1988, and Sakai et al., "Frequency Stabilization of Laser Diode Using a Frequency-Locked Ring Resonator to Acetylene As Absorption Lines," IEEE Photon. Technol. Lett., vol. 3, pp. 868-870, Oct. 1991.) These known techniques provide lasers that transmit a comb or set of equally spaced absolute reference frequencies. However, because each transmitter laser set employ a different resonator that cannot be guaranteed to produce an identical set of resonant frequencies, the channel spacing will differ slightly from one WDM system to another and hence each WDM system will operate at a different comb of frequencies.
Another known technique for maintaining a constant channel spacing is disclosed in R. Boucher et al., "Calibrated Fabry-Perot Etalon as an Absolute Frequency Reference for OFDM Communications," IEEE Photonics Technol. Lett., vol. 4, pp. 801-804, July 1992. This reference employs piezoelectrically tuned Fabry-Perot interferometers calibrated in the 1.3 micron spectral region. By adjusting the cavity lengths identical sets of resonant frequencies can be provided. The interferometers are then locked to an absolute reference to maintain the calibrated cavity lengths. However, the interferometers adjusted by this procedure cannot maintain their calibration without continuous feedback so that they remain locked to the absolute reference.
Accordingly, there is no known simple and reliable technique for synchronizing etalons so that different etalons can provide identical sets of equally spaced frequencies.